Fiber-reinforced magnesium oxychloride bond

ABSTRACT

Magnesium oxychloride cement bonds reinforced with selected fibers have improved resistance to cracking and disintegration in use. Particular embodiments include fiber-reinforced abrasive tools based on these bonds which have improved bond integrity in dry-grinding applications.

This application is a divisional of application Ser. No. 08/388,885filed on Feb. 14, 1995, now U.S. Pat. No. 5,571,317, and applicationSer. No. 08/099,415, filed Jul. 30, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of Art

Magnesium oxychloride cement has been used as a bonding agent sinceearly in the twentieth century. While processes for preparing magnesiumoxychloride cement bonds are well-known in the art, the chemistry ofthese processes is not fully understood to this day. The cement is acomplex reaction product of calcined magnesium oxide, magnesiumchloride, and water, which is admixed in standard proportions andallowed to cure or harden into a cement variously designated in theindustry as "magnesite cement", "magnetic cement", "Sorel cement","French cement", and other terms. "Magnesium oxychloride cement bond"and "magnesite bond" are used interchangeably herein.

Curable magnesium oxychloride cement compositions broadly range fromcastable mixes to mixes that are quite stiff and are tamped into molds.While the invention herein is applicable to compositions encompassing atleast this broad range of formulations, it is particularly applicable tothe latter compositions, those that are stiff, and tamped into a moldsuch as a wheel mold, wherein they are cured to provide the desiredshaped product.

Magnesium oxychloride cement has particular application in theproduction of abrasive tools used for grinding, especially abrasivewheels. The wheels are typically prepared by admixing the bondprecursors with abrasive grit and optional additives, molding themixture into a wheel shape by the above-described tamping operation, andcuring the mixture to harden the bond components into a magnesiumoxychloride cement bond containing abrasive grit material of a type andamount suitable for the contemplated grinding application.

The product has grinding characteristics eminently suitable for drygrinding applications. Unfortunately, the magnesium oxychloride cementbond is relatively brittle, with a low coefficient of thermal expansion,and the strength of this bond can rapidly deteriorate under the heatgenerated during dry grinding. The cause of this is not clear;dehydration of the magnesite bond at elevated temperatures may be atleast partially responsible. Whatever the etiology of this phenomenon,cracking of the bonding cement during grinding may occur, particularlyafter prolonged constant use. Such cracking is often followed bypremature disintegration of the wheel.

2. Discussion of Related Art

Traditionally, liquid coolants, often fortified with lubricant material,are employed to dissipate heat engendered in various other grindingprocesses to protect both the abrasive tool and brittle or otherwisesusceptible substrates from heat damage. Wet grinding applicationsemploying liquid coolants are not, however, suitable for use inconjunction with magnesium oxychloride cement-bonded abrasive tools.Liquid coolants used to dissipate heat build-up during grinding withabrasive tools not based upon magnesite cement nearly always containwater. Since water softens and dissolves the magnesium oxychloride bondand causes rapid deterioration of the tool, these coolants cannot beused in conjunction with magnesite cement-bonded abrasive tools.Further, certain industrially-important substrates, notably theabove-mentioned coil springs, do not lend themselves to wet grindingprocesses, and so such substrates are commonly dry-ground. As a result,magnesium oxychloride cement-bonded abrasive wheels or other abrasivetools are used almost exclusively for dry grinding applications,particularly for dry-grinding of water-sensitive substrate material suchas coil springs, and the tools are at risk of developing deep cracksunder the heat generated during grinding, followed by disintegration ofthe tool.

The incorporation of a wide variety of materials into cementitiouscompositions to reinforce the cured bond is known. However, few of thesematerials have proved useful in magnesium oxychloride cement bonds, forgood reason.

It has been known for the decades that magnesium oxychloride cement hasbeen in use as a bonding agent for abrasives in grinding applicationsthat the material is highly sensitive to the incorporation of extraneousmaterials, which in additive-effective amounts typically have been foundto weaken bonding of the cement precursors to the point of providing acommercially useless product. As noted above, the chemistry of themagnesium oxychloride cement bonding mechanism is not clearlyunderstood, and addition of extraneous materials to the basiccombination of calcined magnesium oxide, magnesium chloride, and waterwith a view toward improving the formulation has of necessity proceededon an ad hoc basis. Many reinforcing materials have been suggested forstrengthening hydraulic cement bonds other than magnesite bonds such asthose based on Portland cement, as well as vitrified bonds and resinousbonds such as phenol formaldehyde-based resins to promote structuralstrength of the product. However, in combination with the presentmagnesium oxychloride cement precursors, such additives have typicallyproved to be deleterious to the cured bond, often weakening the bondstructure of the product to the point of uselessness for the intendedapplication. Bonds exposed to unusual stress in use, most especiallyabrasive tools for dry-grinding applications, are particularly sensitiveto added materials. Thus, the use of materials and processes known inthe prior art for reinforcing magnesite bonds has been generally limitedto bonds subjected to little or no stress. Construction materials basedon reinforced magnesite cements as described in U.S. Pat. Nos. 3,320,077to Prior et al.; 3,607,825 to Shannon; 4,033,752 to Isohata et al.; and5,049,197 to Leroux et al. and are exemplary.

Theoretically useful reinforcing materials for obviating cracking anddisintegration of magnesite cement bonded abrasive tools such as wheelshave often proved disadvantageous in practice. One proposed reinforcingmaterial for magnesite bonds is wire mesh, which has been incorporatedinto the bond to bridge across the matrix and resist cracking of theproduct under stress. However, if the product is to be used in abrasiveapplications, for example an abrasive wheel, the wire mesh cannot bepresent on a grinding surface of the wheel as wire of sufficient gaugeto function as an effective reinforcement will interfere with thegrinding process. Since the large majority of the wheel is consumed ingrinding, the effectiveness of the wire mesh as a reinforcing agent inabrasive tools is limited. Other known reinforcing materials have provedto seriously compromise the heat strength of magnesite bonds, leading tocracking and disintegration of the bond when exposed to heat, such as indry grinding applications.

SUMMARY OF THE DISCLOSURE

The invention accordingly comprises an improved magnesite cement bondincluding a fibrous component characterized as described below in anamount sufficient to reinforce the bond against stress forces. Inparticular, the invention comprises an abrasive composition includingmagnesium oxychloride cement precursors, abrasive grit, and the fibrouscomponent, curable to provide a magnesite-bonded abrasive tool,especially an abrasive wheel, resistant to cracking and/ordisintegration in use, particularly under heat exposure in dry-grindingapplications. The selected fibers provide reinforcement throughout theentire matrix of the bond, increasing the life and safety of the productby bridging minor fractures in the bond and resisting the development ofthese fractures into deep cracks which might compromise the integrity ofthe bond. The reinforcing fibers according to the invention do notsignificantly weaken the magnesite bond chemistry, do not affectgrinding efficiency, are dispersable substantially evenly throughout thematrix of the bond to ensure uniform strength of the product, and in aparticular embodiment, are combined with an adhesive to promoteadherence of the fibers within the matrix of the bond under stress. Theproduct is an exceptionally useful dry-grinding tool, especially anabrasive wheel.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, a fibrous component is added to aconventional magnesium oxychloride cementitious bonding compositionessentially comprising calcined magnesium oxide, magnesium chloride, andwater to form a plastic, settable composition suitable for the intendeduse. Formulations for such settable magnesium oxychloride cementitiouscompositions are well-known in the art; the present invention isdirected to the structural improvement in the hardened final productsafforded by the presence of the fibrous component. While abrasive toolscomprising fiber-loaded magnesium oxychloride cement bonds containingabrasive grits are particularly contemplated, especially for theresistance of the product to the development of significant cracks dueto the heat generated in dry-grinding applications, the strengtheningqualities of the fibers in combination with conventional magnesiumoxychloride cement bonds is also useful in applications other thangrinding applications.

Fibers suitable for incorporation with the magnesium oxychloride cementprecursors (i.e., the plastic, settable magnesium oxide/magnesiumchloride/water admixture, herein also referred to as the "magnesitecementitious composition") broadly comprise fibers of a length andnumber to provide substantial bonding of the fibers within the bondmatrix on both sides of any early hairline fractures in the bond so thatwidening of the cracks is substantially prevented. The exact length ofthe fibers will vary according to the degree of reinforcement deemednecessary to maintain the integrity of the bond in a given application;however, for adequate reinforcement of heavy magnesite-bonded abrasivetools such as abrasive wheels, a minimum average length of about 1/8" isdeemed to be critical; the maximum length is not critical, however,fibers between about 1/16" and about 2" in average length arerecommended for most applications. The amount of the fibers is similarlyvariable, but again, for the abrasive wheels of the invention, fibers inthe amount of from about 0.2% to about 10% by volume, preferably about0.2% to about 5%, and most preferably about 0.5% to 3%, of the curablecementitious magnesite composition is highly preferable for mostapplications.

Fibers useful in the practice of the invention comprise those fibershaving a breaking strength which can withstand the forces applied by acracking matrix in order to intercept further development of thecrack(s). Substantially non-stretchable fibers are preferred. Metalfibers, synthetic resin fibers, ceramic fibers and glass fibers arecontemplated: stainless steel, carbon steel, brass, bronze, copper,aluminum, carbon; polyesters, polyamides such as the various nylons,polyaramides, polyolefins such as polypropylenes, or copolymers thereof;or fiberglass, are all useful, providing that their physical propertiesconform to the above-described parameters, and that the amount of thefibers required for sufficient reinforcement of the matrix, or theproperties thereof, do not substantially adversely affect the chemistryof the magnesite bond. Fiber diameter necessary to fulfill the breakingstrength requirement will depend upon the particular fiber selected;however, an exemplary material comprises fiberglass having an averagelength of from about 1/8" to about 2" in length, preferably about 1/4"to 3/4", and a diameter of from about 5 to 50 microns. Fiber diameter inan abrasive composition should not be large enough to impact upon thegrinding process contemplated; in general, the diameter of non-metalfibers such as fiberglass can be of broader range, as these fibers arereadily consumed in the grinding process without affecting the workpiecesurface.

It is essential to the invention that the chosen fibers be readilydispersible in the cementitious composition to ensure substantiallyuniform dispersion of the fibers throughout the bond matrix. Singlestrand fibers, particularly single strand fiberglass fibers areunsuitable for the present invention, as when admixed with the magnesitecement precursors, the strands "bird nest", creating islands of fibersin the cured matrix, rather than the substantially uniform distributionof fibers requisite for achieving the desired results of the invention.Surprisingly, it has been found that introducing the fibers as bundlesthereof, rather than as single strands, into the characteristicallyviscous and heavy magnesite cement mix results in efficient dispersal ofthe fibers within the mix to provide the desired uniformity.Accordingly, fiber bundles, especially bundles that separate duringmixing to provide both individual and bundled reinforcing units, areemployed in the practice of the invention; fiberglass bundles areparticularly suitable. Exemplary fiber bundles comprise choppedfiberglass bundled strands available from PPG, Pittsburgh, Pa., USA.

In a particularly advantageous embodiment of the invention, the fiberbundles are precoated with an adhesive capable of bonding the fibers tothe magnesite matrix. The adhesive is selected according to the fibercharacteristics, and the bond characteristics, including thecharacteristics of the abrasive grit or other filler(s) present, topromote secure embedment of the fibers within the bond and to enhanceresistance of the fibers to being dislodged from the matrix of the bond.Epoxy resin adhesive has proven particularly effective for adheringglass fibers to abrasive-loaded (e.g., aluminum oxide and\or siliconcarbide abrasive grit) magnesite bond; an exemplary useful epoxyadhesive Epi-Rez WD-510, is available from Rhone-Poulanc, Louisville,Ky., USA. In general, the adhesive is incorporated into the compositionin adhesive-sufficient amounts, suitably by admixing the fibers with theadhesive prior to incorporating the fibers into the magnesitecementitious composition; recommended proportions of fiber to adhesiveare from about 5% to about 180% by wt. adhesive, based on the weight ofthe fiber; the range will vary, depending, inter alia, upon the selectedfiber and adhesive. For glass fibers, an amount of epoxy adhesive (resinplus hardener in conventional proportions) of from about 30% to about100% more typically from about 60% to 80%, by weight of adhesive basedon the weight of fibers is exemplary. Preferably, the adhesive iswater-miscible to facilitate a homogenous mix.

The magnesite cementitious composition is readily prepared fromcommercially available materials as known in the art. Magnesium oxidepowder is typically combined with an aqueous solution of magnesiumchloride in proportions which give the desired consistency to the mixand the desired product. Exemplary useful ingredients includesubstantially pure calcined magnesium oxide powder admixed with anaqueous solution of magnesium chloride of, for example, from about 15 toabout 30 wt. % magnesium chloride, in amounts of about equal parts byweight of the solution and powder up to an excess of the magnesiumchloride solution of about 10 to 15% by weight of the calcined MgO. Thisadmixture is supplemented with water as necessary to improve theconsistency of the cementitious mix. Particularly useful sources ofmagnesium chloride for the magnesite cementitious compositions of theinvention are solutions marketed by Dow Chemical, Midland, Mich. USAsuch as L-30 (aqueous magnesium chloride, 30° Baume).

Other components can be added to the mix as desired, insofar as suchadditives do not substantially weaken the magnesite bond. Any suchadditives are generally selected according to the intended use of theproduct. For example, in applications requiring substantial flexuralstrength, such as large abrasive grinding tools, various clays such asaluminum-silicate clays, especially kaolin clays, are frequently addedas strengthening agents. For abrasive tool applications, suitableabrasive grits such as silicon carbide, aluminum oxide, sol-gel aluminumoxide, or glass frit are incorporated as customary in the art. Pigments,as another example, may also be included wherein visual qualities are ofimportance. Additives such as fillers and grain spacers are furtherexemplary, with the proviso that any such additives do not substantiallycompromise the integrity of the magnesite bond. In certain toolapplications, such as abrasive wheels, the mold may be prepared withinserted nuts, cups, dowel caps, wire mesh or other elements known inthe art.

For general use, the fiber component (optionally adhesive-coated) isconveniently admixed with the dry ingredient(s), and the remainingliquid cement precursors incorporated according to methods recognized inthe art. Typically, any dry additives such as clay, abrasive, or pigmentpowders are additionally first admixed with the magnesium oxide powderand fibers, followed by gradual incorporation of the liquid ingredients(including magnesium chloride if added as aqueous solution) with mixingto form a homogeneous (non-lumpy) mass. The mixture is then shaped andcured in customary fashion. If adhesive-coated fiber is incorporated asdescribed above, curing conditions are adjusted as necessary to cure theadhesive as well as the magnesite bond precursors.

In a particular embodiment of the application, fiber is incorporatedinto a magnesium oxychloride cementitious material formulated for thepreparation of an abrasive tool, especially an abrasive wheel. In anexemplary preparation, for any given conventional abrasive toolformulation, fiber as described above, especially glass fiber from about1/16" to 1.5", particularly about 1/8" to 1", and especially about 1/2",in average length in bundles is coated with a conventionalwater-miscible epoxy resin adhesive system comprising epoxy resin andhardener. Calcined magnesium oxide, abrasive grit such as fused aluminumoxide or silicon carbide in an amount sufficient for the intendedapplication, and any other dry ingredients are then combined with theadhesive-coated fibers. Magnesium oxychloride solution is then added tothe dry mix with further thorough mixing; water or additional magnesiumoxychloride solution may be added to the resulting cementitiouscomposition to adjust the consistency thereof in customary fashion.Alternately, a conventional magnesite cementitious composition isadmixed, followed by addition of the epoxy-coated fibers with furthermixing to provide a homogenous mass.

The product composition is then placed, for example, into a wheel mold,and compacted by a usual method. The mold may be prepared prior to useby, for example, insertion of nuts as known in the trade for anut-inserted abrasive disc product. Other preparations of the molds,including those described supra, may be employed. The molded product iscured under controlled conditions to transform the bond precursors intoa magnesium oxychloride bond and to cure the epoxy adhesive precursors,for example, for about 10 to 75 days, usually from about 10 to 60 days,at a temperature of from about 90° to 110° F.

If desired, the magnesite bond may include other reinforcing agents inaddition to the described fibers, such as wire mesh preset into the moldbefore pouring of the uncured material.

The following Examples are illustrative of the invention.

EXAMPLE I Materials

White or brown aluminum oxide abrasive grit.

Water miscible epoxy adhesive system (Epi-Rex WD 510, Rhone-Poulenc,Louisville, Ky., USA).

Calcined magnesium oxide powder.

Magnesium chloride (29% aqueous solution 30° Baume, Dow Chemical,Midland Mich., USA).

Fillers (Kaolin clay, Georgia Clay Co., GA, USA).

Glass fibers (Certain Teed Fiberglass Reinforcements, Wichita Falls,Tex., USA; CT 919-A4, 1/2" length.

A composition was prepared of the above materials as follows:

    ______________________________________                                        Ingredient         Amount (oz.)                                               ______________________________________                                        Al.sub.2 O.sub.3   283.18                                                     Epoxy resin        3.35                                                       Epoxy resin hardener                                                                             1.12                                                       MgO and Kaolin Clay                                                                              46.84                                                      (100 parts MgO to 25 parts clay)                                              MgCl.sub.2 aqueous solution                                                                      42.87                                                      Glass fibers (1/8" to 13/4")                                                                     6.38                                                       ______________________________________                                    

The glass fiber bundles (constituting 2% by volume of the abovecomposition) were mixed in a first mix bowl with the epoxy adhesivecomponents for 1 minute. In a second mix bowl, the abrasive grit,magnesium oxide, filler, and magnesium chloride solution and water weremixed for 3 minutes. The contents of the first mix bowl were added tothe second mix bowl and the combined ingredients mixed for an additional3 minutes to homogenize the mix. A portion of the resulting final mixwas evenly distributed in a conventional 36" steel wheel mold providedwith mounting nuts, compacted, and cured at 100° F. for 30 days toobtain a nut-inserted abrasive disc.

EXAMPLE II (COMPARISON EXAMPLE)

A control wheel was prepared as described in Example I, except that theepoxy adhesive system and glass fibers were omitted, and the remainingingredients were mixed for a total of 3 minutes to homogenize.

A. Both wheels were heated in excess of 350° F. on the grinding faces ina radiant heat oven. Heat cracks appeared in the unreinforced wheelafter approximately 1/2 hour due to internal stresses caused byexpansion and evolution of volatiles. The fiber reinforced wheel ofExample I showed no heat cracking under the same conditions.

B. The fiber-reinforced wheel was then allowed to cool and wasdeliberately cracked in a hydraulic press along intersecting directions.The wheel was mounted in a testing machine which rotated the wheel athigh speed to simulate centrifugal forces encountered during grindingoperation. The wheel was mounted by bolting the wheel to a steel plateusing the mounting nuts in the wheel back. The wheel was thenaccelerated to 1.5 times its normal operating speed, equivalent to 8250surface feet per minute or 875 rpm on a 36" diameter. Under theseconditions, 2.25 times the centrifugal stress exerted at normaloperating speeds was placed on the wheel. No visible damage wasobserved, and the wheel remained intact. The cracks did not enlarge.

C. A pie-shaped cracked piece of the stressed wheel from B, above,approximately 1 foot square was then targeted for testing. The piece waslocated on the outer diameter of the wheel where it was not constrainedby the rest of the wheel. All mounting bolts were then removed from thetest piece, and the wheel was again accelerated to 875 rpm. Despite thecentrifugal stress placed on the test piece, the test piece did notdetach from the body of the wheel, but remained in its originalposition. It was concluded that the fibers held the pie-shaped crackedpiece in place, without assistance of mounting nuts.

EXAMPLE III (Effect of Fibers on Magnesite Bond Strength)

Bars were made using the control composition and various fiberreinforced compositions (Table). After curing, all were broken in auniversal strength tester in a 3 point transverse mode. The flexuralstrength (primary) of all the non-reinforced bars was recorded, leavingall the non-reinforced bars in pieces. The maximum flexural strength ofthe reinforced bars was also recorded ("primary"); however, all thereinforced bars remained in one piece. The test on the reinforced barswas continued by allowing the head of the universal tester to continuedownward onto the bars until ultimate failure of the fibers occurs. Thissecond flexural strength ("secondary") is recorded as the strength ofthe fiber reinforcement surviving the matrix cracking as shown in Table1:

                  TABLE 1                                                         ______________________________________                                        Fiberglass*                                                                   Content   Epoxy**  Flexural Strength                                                                             % Strength                                 (% of Wt. Vol)                                                                          Content  Primary   Secondary                                                                             Retained                                 ______________________________________                                        control    0       3236   psi  0    psi  0%                                   1.0       50       3212        1079      33.6                                 2.0       50       3186        2099      65.6                                 0.5       60       3181        306        9.6                                 1.0       60       3233        1217      37.6                                 1.5       60       3372        2108      62.5                                 2.0       60       3368        1689      50.1                                 1.0       70       3168        1235      39.0                                 2.0       70       3478        2237      64.3                                 ______________________________________                                         *Fiber content as a percent of the wheel volume.                              **Epoxy and hardener content as a percent of fiber weight.               

In all cases, the fiber-reinforced bars show a significant secondarystrength not present in the control bars. Also, all fiber-reinforcedbars show a primary strength that is very close to or superior to thecontrol. The optimum case shown is from example 1 above, which contains2% fiberglass by volume and 70% of the fiberglass weight as epoxy andhardener. In this case, the bars are 7% stronger than the controlinitially and retain 64.3% of their strength after the matrix iscracked. By comparison, the control bars had no strength after thematrix cracked and they fell into pieces.

Although the preferred embodiment has been described in considerabledetail through the above examples, these are presented for illustrationonly. Variations and modifications can be made by one skilled in the artwhile keeping within the spirit and scope of this invention.

What is claimed is:
 1. A method for preparing a reinforced magnesiumoxychloride cement bonded abrasive tool comprising incorporatingabrasive grit and ceramic fibers having a minimum average length of atleast about 1/4" into an uncured magnesium oxychloride cementitiouscomposition in an amount sufficient to reinforce the cured bond againstdisintegration in use, followed by compacting and curing of thecomposition, to form the abrasive tool.
 2. The method of claim 1,wherein the fibers are glass fibers.
 3. The method of claim 1, whereinthe fibers are incorporated into the uncured cementitious composition inthe form of bundles, followed by mixing to provide a curable compositioncontaining individual fiber strands and bundled fiber strands uniformlydistributed throughout the composition.
 4. The method of claim 3,wherein the fibers are glass fibers.
 5. The method of claim 3, whereinthe fibers are incorporated into the uncured composition in the form ofadhesive-coated bundles.
 6. The method of claim 5, wherein the adhesiveis an epoxy adhesive.
 7. The method of claim 5, wherein the fibers areglass fibers.
 8. A method for dry grinding a workpiece, comprisingabrading the workpiece with an abrasive tool comprising a magnesiumoxychloride cement bond containing ceramic fibers having a minimumaverage length of at least 1/4" in an amount sufficient to reinforce thebond against disintegration in use.
 9. The method of claim 8, whereinthe fibers are glass fibers.
 10. The method of claim 8, wherein the bondis prepared from a curable cementitious composition comprising magnesiumoxide, magnesium chloride, water, and abrasive grit; and ceramic fibersin an amount from about 0.2 to about 10% by volume of the composition.11. The method of claim 10, wherein the fibers are glass fibers.
 12. Themethod of claim 11, wherein the abrasive tool is an abrasive wheel. 13.The method of claim 8, wherein the fibers are incorporated into thecementitious composition in the form of bundles.
 14. The method of claim13, wherein the fibers are fiber glass bundles.
 15. The method of claim8, wherein the fibers are adhered within the bond with an adhesive. 16.The method of claim 15, wherein the adhesive is a water-miscible epoxyadhesive.
 17. The method of claim 15, wherein the bond is prepared froma curable cementitious composition comprising magnesium oxide, magnesiumchloride, water, and abrasive grit; and ceramic fibers in an amount offrom about 0.2 to 10% by volume of the composition.
 18. The method ofclaim 17, wherein the fibers are incorporated into the cementitiouscomposition in the form of fiber bundles coated with adhesive.
 19. Themethod of claim 18, wherein the adhesive is an epoxy adhesive.
 20. Themethod of claim 19, wherein the adhesive is a water-miscible epoxyadhesive.